1. Field of the Invention
The present invention relates to a hydrostatic transaxle (hereinafter referred to as an “integrated hydrostatic transaxle” or as “IHT”) in which a common housing incorporates a hydrostatic transmission (hereinafter referred to as “HST”), an axle, and a deceleration drive train interposed between the HST and the axle, wherein the HST includes a hydraulic pump and a hydraulic motor fluidly connected to the hydraulic pump.
The present invention also relates to a center section for mutually fluidly connecting a hydraulic pump and a hydraulic motor so as to constitute an HST which is adaptable to the IHT.
2. Related Art
As disclosed in U.S. Pat. Nos. 5,156,576; 6,390,227; 6,397,594; and 7,043,908 (hereinafter referred to as references '576, '227, '594 and '908), there is a well-known IHT in which a common housing incorporates a HST, an axle, and a deceleration drive train interposed between the HST and the axle. The HST includes a pair of hydraulic displacement units serving as a hydraulic pump and a hydraulic motor. The HST also includes a center section. A pump cylinder block of the hydraulic pump and a motor cylinder block of the hydraulic motor are mounted on the center section so as to fluidly connect the hydraulic pump and motor to each other through fluid passages formed in the center section.
In the conventional IHT, the housing includes two distinct housing members separably joined to each other. For example, as disclosed in references '576, '227 and '908, the two housing members are upper and lower housing members joined to each other through a horizontal joint surface. Otherwise, as disclosed in reference '594, the housing members are left and right housing members joined to each other through a vertical joint surface.
Conventionally, both of the housing members cooperatively support the HST and the deceleration drive train. In this regard, with respect to the IHT including the upper and lower housing members, as disclosed in references '576, '227 and '908, a motor shaft of the hydraulic motor is disposed so as to have a center axis thereof on the joint surface between the upper and lower housing members, thereby being supported by the upper and lower housing members. During rotation of the motor shaft, both the upper and lower housing members are loaded with the thrust or radial force generated from the motor shaft. Further, a retainer of a swash plate (a thrust bearing) of the hydraulic motor is sandwiched between the two housing members, i.e., supported by the two housing members. The thrust forces of pistons fitted in the motor cylinder block against the swash plate are transmitted to both of the housing members through the retainer. Further, as disclosed in references '227 and '908, an intermediate (counter) shaft of the deceleration drive train is disposed so as to have a center axis thereof on the joint surface between the two housing members, thereby being supported by the two housing members so that both the housing members are loaded with radial or thrust force caused by rotation of the intermediate shaft.
With respect to the IHT including the left and right housing members, as disclosed in reference '594, a motor shaft of the hydraulic motor is supported at a first end thereof by one of the left or right housing members. The other of the left or right housing members supports a thrust bearing serving as a swash plate against which pistons fitted in a motor cylinder block abut, and a second end of the motor shaft is disposed in the motor cylinder block. Thus, the HST is supported by both the left and right housing members. Further, an intermediate shaft of the deceleration drive train is supported at opposite ends thereof by the respective left and right housing members. As a result, both the left and right housing members are loaded by the HST and the deceleration drive train.
To economize the housing, it is suggested that one of the housing members is made of an inexpensive material, such as a plastic or a synthetic resin. However, in the conventional IHT mentioned above, both of the housing members must support the HST and the deceleration drive train. In other words, both of the housing members are loaded by the HST and the deceleration drive train. Ideally, housing members to be made of inexpensive material should be free from such loads.
In addition, to compact the IHT, narrowing the center section is desirable. Further, if the hydraulic pump is variable in displacement, disposing a pump control arm and its complex surrounding mechanism (which are exposed on an outside surface of the housing) in a dead space is desirable in that it prevents this structure from interfering with another external mechanism.
In this regard, as disclosed in references '227 and '594, the center section is suggested to have a narrow portion offset from its pump mounting portion, so as to support the motor shaft and the motor cylinder block. The motor cylinder block is provided on one end portion of the motor shaft on a first side of the center section, and a motor gear constituting the deceleration drive train is provided on the other end portion of the motor shaft on a second side of the center section opposite to the first side. However, the pump control arm is disposed on the second side of the center section adjacent to the motor gear. The portion of the housing on the second side of the center section occupies a large space for containing gears or elements of the deceleration drive train, thereby hindering the pump control arm from being compactly arranged.
Further, to economize the center section, simplification of boring the fluid passages in the center section is desirable. Conventionally, the fluid passages formed in the center section to be interposed between the pump cylinder block and the motor cylinder block include holes bored to connect pump kidney ports to motor kidney ports, thereby complicating the machining process.